How We Lost a Gravitational Wave Source and Gained a Supernova

What started as the search for the source of a potential gravitational wave signal ended with the discovery of an unusual supernova. The supernova, SN2019wxt, showed a double-peaked light curve similar to previous ultra-stripped supernova candidates.

There and Gone

plot showing the location of the newly discovered transient

Location of the newly discovered transient, labeled AT2019wxt, in the outskirts of its host galaxy. [Shivkumar et al. 2023]

In December 2019, the LIGO and Virgo gravitational wave detectors distributed an alert for an event cataloged as S191213g, jump-starting a search for an electromagnetic counterpart to the possible gravitational wave signal. In the days following the alert, multiple telescopes turned toward the source region for the signal, homing in on a rapidly evolving object that was brightening the outskirts of a compact galaxy about half a billion light-years from Earth. Further analysis of S191213g downgraded its significance as a gravitational wave signal, ending the search for its source — but the newly discovered object got even more interesting.

“Just” a Supernova

In a recent research article, Hinna Shivkumar (University of Amsterdam) and collaborators outlined the follow-up observations of this intriguing target. As early data trickled in, the object remained hard to classify, though its mostly featureless spectrum with a broad emission line from helium marked it as an exploding star that had lost its outer layers of hydrogen, and it gained the label SN2019wxt.

optical and near-infrared light curve of the event

Optical and near-infrared light curves of SN2019wxt over three weeks following the initial detection. The i and g bands show the intriguing double-peaked shape. Click to enlarge. [SN2019wxt et al. 2023]

Shivkumar and coauthors used X-ray data from the Chandra X-ray Observatory, radio data from the Very Large Array, and optical images and spectra from telescopes across several continents to study the explosion further. Rather than showing a single peak to its light curve like a typical supernova, SN2019wxt peaked twice in just three days, making it one of the fastest-evolving supernovae known. Modeling of SN2019wxt’s light curve suggested that the first peak is due to rapid cooling of an expanding bubble of plasma, and the second peak is due to radioactive decay of material ejected in the explosion.

Double Peaked and Ultra-stripped?

Bolometric light curve compared to best-fitting models

Bolometric light curve of SN2019wxt (black circles) and best-fitting models of shock cooling (green dashed line) and radioactive decay (blue dashed line). [Shivkumar et al. 2023]

The unusual light curve, lack of hydrogen spectral lines, and modeled ejecta mass and explosion radius place SN2019wxt as a possible ultra-stripped-envelope core-collapse supernova. This rare class of supernovae contains only a few candidates, which are characterized by rapidly declining brightness, double-peaked light curves, and the presence of circumstellar material. These features point to stars that are stripped of much of their mass before exploding, leaving little material to be ejected in the explosion.

The serendipitous discovery of SN2019wxt makes for a great story, but to learn more about ultra-stripped supernovae in the future, we’ll need to catch them right when they happen. Luckily, the Vera C. Rubin Observatory’s long-awaited Legacy Survey of Space and Time draws ever closer, and after its anticipated start in 2025 will bring one million supernova detections each year — and thus millions of opportunities to study rare supernovae like SN2019wxt.


“SN2019wxt: An Ultrastripped Supernova Candidate Discovered in the Electromagnetic Follow-up of a Gravitational Wave Trigger,” Hinna Shivkumar et al 2023 ApJ 952 86. doi:10.3847/1538-4357/acd5d5